Many circuits often employ power amplifiers, such as RF power amplifiers in handheld communication devices and other wireless devices. Power amplifiers are often controlled by adjusting a supply voltage VCC provided to the power amplifiers. A low drop out (LDO) linear amplifier is routinely used to generate the supply voltage VCC for a power amplifier.
Linear amplifiers can be very efficient when used to generate the supply voltage VCC for a power amplifier. For example, when the supply voltage VCC is very close to the power supply of a linear amplifier (often a battery voltage VBATT), the linear amplifier is typically very efficient. However, when the supply voltage VCC is far below the battery voltage VBATT, the linear amplifier may be very inefficient.
Linear amplifiers can also be designed to have extremely low noise and very high bandwidth. The low noise property is often important because there can be strict linearity requirements placed on power amplifiers, and any noise in the supply voltage VCC can translate into power amplifier output noise. The high bandwidth property is also often important because there can be spectral mask and time mask requirements in different communication systems, such as when the supply voltage VCC needs to be fully ramped up or down rapidly (such as within 10 μs) and when the supply voltage VCC needs to maintain its modulation dynamics.
Switching converters can provide high efficiency, but they typically have limited bandwidth. This may be due to several reasons, such as high switching losses at high switching frequencies, low resonant frequencies of external inductor-capacitor (LC) components, and control difficulties. Also, switching converters typically produce switching noise at the desired switching frequencies. As a result, typical switching converters are inappropriate for use in generating a supply voltage VCC requiring envelope tracking or short time masks for a power amplifier.